1. Field of the Invention
The present invention relates to an improved enhanced oil recovery method and, more particularly, to such a method for use within a micellar flood program.
2. Setting of the Invention
It is well known that in micellar flooding the surface active agent or surfactant has a tendency to adhere to the rock media's surface as well as to the interface between the oil and water and be lost from the fluid. This surfactant loss or retention, which greatly reduces the effectiveness of the micellar flood, is caused by adsorption, phase trapping, and sulfonate-polymer incompatibility. The sulfonate loss depends primarily on the ionic conditions, i.e., salinity and hardness, of the polymer water and not of the micellar fluid. In particular, sulfonate loss depends on the in situ generated compositions at the trailing edge of the micellar fluid where mixing with polymer water occurs. Various methods and procedures have been suggested for optimizing the micellar flood process. For example, U.S. Pat. No. 3,343,597 suggests preceding the injection of an oil miscible micellar dispersant by injecting an "insulating" water slug having an ionic content matching that of the water and the micellar dispersion. Also, U.S. Pat. No. 3,467,190 suggests that injecting an aqueous surfactant slug having an optimized salt concentration for providing a minimum interfacial tension against a reservoir oil and preceding or following that slug with an aqueous solution of the same salt concentration whenever the salt concentration of the formation water or drive water differs from the optimum concentration. U.S. Pat. No. 3,482,631 suggests that where an oil-displacing fluid contains an electrolyte, the adsorption of the electrolyte when the equilibrium is established at the junction between the injection fluid and the reservoir water is reduced by injecting a preflood solution having a selected electrolyte concentration of an effective viscosity at least equaling that of the reservoir fluid. U.S. Pat. No. 3,648,700 suggests determining which cation is predominant in the formation water and injecting a micellar dispersant containing a petroleum sulfonate and a cation which has a greater affinity for the sulfonate than the predominant cation in the reservoir water. U.S. Pat. No. 3,915,230 suggests preceding an aqueous surfactant system of optimum salinity and hardness with a thickened aqueous preflush solution of the same salinity and hardness. Patents listed above indicate that where the formation water is highly saline or contains a relatively large multivalent cations, it may be impossible, or at least uneconomical, to formulate an optimum surfacant slug having the ionic composition of the formation water. U.S. Pat. No. 4,074,755 teaches that a salinity contrast in the micellar flooding may be used. This patent suggests that the micellar flooding process should be designed such that the calcium-to-sodium ratio is identical in all brines in order to eliminate ionic exchange.
In actual operation, it has been found that even by utilizing the procedures of the patents listed above, significant surfactant loss still occurs. Furthermore, unfavorable ion exchange is not eliminated by maintaining identical calcium-to-sodium ratios in all brines. This is because the surfacant itself initiates an ion exchange process.